Tristan Lear, 39. baboon males often approach each other and gently grasp each other's penises. Primatologists refer to this behavior as "diddling." A tension reducing gesture, akin to the human handshake, it reciprocally assures that neither party has aggressive intentions. Ancient Romans would conclude a transaction by placing their testicles in each other's palm while mutually reciting an oath - the ultimate expression of trust. The words "testimony" and "testament" both derive from this ancient practice.